The present invention relates to the measurement of a delay in a delay circuit by using a continuous frequency measurement.
There are many ways to measure a delay in a delay circuit. The conventional method of time interval measurement is a "start and stop" method. In this patent a measurement of a time delay between a start event and a stop event is made by counting the number of pulses of a master-clock signal which occurred between the start and stop events. This results in quantization of the measurement to 1 clock period. To measure time interval with resolution below 1 clock period requires special precision circuitry. See U.S. Pat. No. 4,164,648 which describes the double vernier time interval measurement using triggered phase-locked oscillators. Attempts to improve the resolution is usually expensive, because it needs an expensive high speed technique to very quickly capture both the start and the stop signals.
To measure short time durations, this method produces the high fractional error which is the quantization error divided by the time duration. Indeed, the master clock signal has quantization error connected with the (+1) or (-1) count ambiguity. The quantization error is equal to the period of the time-based clock. So, for a 100 MHz clock signal the quantization error is equal to 10 nsec. To measure a 100 nsec time interval, the fractional error is 10%.
Besides the quantization error, there is also a systematic measurement bias. Indeed, the start event is measured by the start channel trigger circuitry, and the stop event is measured by separate circuitry. Because the conventional technique uses two different measurement circuits for the start and stop events, any mismatch between them produces a systematic measurement bias. The typical systematic measurement bias is about 0.5 nsec. So, the fractional error for the start and stop method is even larger, because both quantization and systematic bias error contribute to the fractional error.
Repeated Start and Stop measurements can be made and the average result can be used to estimate the time interval. Depending on the rate at which these measurements are repeated, the average value may or may not converge to the time interval value. Even in the case where it converges, the convergence is slow and therefore time consuming. See the U.S. Pat. No. 3,938,042.
However, the averaging method does not reduce the systematic measurement bias and therefore the fractional error remains large even with averaging. Thus, it is desirable to provide an inexpensive time measurement method which reduces the fractional error for the measured delay time without having to reduce the quantization and systematic errors of time interval measurement.